Formula zigzag projection

Name all possible stereoregular polymers of oxiranes and give their formulae (zigzag and adopted Fischer projections). Indicate which of these stereoregular polymers are obtainable and which have never been obtained. 

Draw the Fischer projection formula for the sympathomimetic oxilofrine (erythro configuration) and then convert this formula into a zigzag projection. As a check determine the absolute configuration at each step. 

The bromination of cinnamic acid leads to two intermediate enantiomeric cyclic bromonium ions which are subsequently attacked by a nucleophilic bromide ion. Since this attack can occur both at positions 2 and 3, and because the reaction proceeds stereospecifically with inversion, both bromonium ions will give rise to mutually identical pairs of enantiomeric products. This is easier to see from the Fischer projection formulae, which are readily obtained from the zigzag projections by rotation around the C2-C3 bond. 

In order to be able to decide which transformations must be carried out at which groups in the starting molecule it is first of all best to convert the Fischer projection formula of the target molecule to a zigzag projection which corresponds to the orientation of the substituents in the starting material. Once this has been accomplished it is a relatively simple task to identify the necessary reaction steps. 

Fig. 2. Planar zigzag projection of the backbone carbon atoms of a) isotactic and b) syndiotactic polypropylene, both in the saw-horse and in the Fischer notation. It should be noted that the latter notation is not strictly a Fischer projection formula, because, by the Fischer convention, the carbon chain is always written vertically. In addition, only the methyl group positions are...

Scheme 9. The threo 2,3-diol problem. The erythro/threo notation is based on Fischer projection formulas. For example, if threo 2,3-diol 28, shown here in a staggered zigzag conformation, was depicted in an eclipsed Fischer projection, the adjacent hydroxyls attached to carbons 2 and 3 would reside on opposite sides of the carbon chain. An alternative, perhaps less ambiguous, descriptor is the syn/anti notation.63...

The contrast between formulas 20 and 21, both pertaining to isotactic polyethylidene, should be noted This contrast occurs because the polymer repeating unit has only one carbon atom in the chain and thus there is no correspondence between such periodicity and that of the zigzag representation. The classical definition of an isotactic polymer (as one in which all substituents are on the same side of the chain) holds true, in general terms, only if the polymer is represented in the Fischer projection. Analogous considerations pertain to syndiotactic polyethylidene 22 and 23. 

The points marked m in Figure 1(b) represent a pair of jointly projecting A1 atoms the points marked m in Figure 2(b) are the A1 atoms forming a zigzag chain parallel to the a axis. The basic chemical formula of pyrophyllite can be written 8Si02-4A10(0H). The elementary sheet is, therefore, electrically neutral. 

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